HED (Howardites-Eucrites-Diogenites)
HED Group Achondrites - Meteorites from Vesta
This main group comprises three closely related classes of achondrites, known as the HED group, standing for Howardites, Eucrites, and Diogenites. All members of this main group represent evolved achondrites that have experienced extensive igneous processing similar to terrestrial rocks of magmatic origin. Consequently, they closely resemble terrestrial igneous rocks, such as basalts, dolerites, gabbros, and other rocks of volcanic or plutonic origin.
The HED group members share a common oxygen isotope fractionation line as well as other chemical characteristics, indicating that their formation occurred on a common parent body. Their ancient crystallization ages of 4.43 to 4.55 billion years, determined from radiogenic isotopes, suggest that the common parent body has to be a large, differentiated asteroid. This asteroid became geologically inactive after a relatively short but intense igneous history. Early comparisons based on the reflectance spectra of certain meteorite classes and the main belt asteroids yielded the first match - the eucrites with the asteroid 4 Vesta, one of the largest asteroids in our solar system. Ongoing research has confirmed this match for the howardites and diogenites, and today it is widely accepted that 4 Vesta is indeed the common parent for the achondrites of the HED group. Together, the howardites, the eucrites, and the diogenites form the most abundant class of achondrites in our collections.
With a diameter of about 530 km, 4 Vesta is the third-largest asteroid in our solar system. In contrast to the irregular, potato-shaped fragments that characterize most other asteroids, Vesta is a small, nearly spherical, differentiated body that shares many features with the terrestrial planets, i.e. Mercury, Venus, Mars, and Earth. However, Vesta isn't a perfect sphere. In fact, near its south pole we can resolve a 460 km-wide, 30 km-deep crater, the remnant of a huge impact that excavated Vesta's basaltic crust down to its mantle. Due to the immense force of this impact, large chunks of matter were ejected from Vesta forming smaller asteroids of similar composition - the so-called Vestoids. Some of these fragments subsequently entered a near-Earth orbit, and they are thought to be the actual sources of the howardites, the eucrites, and the diogenites that have made their passage to Earth in the form of meteorites.